1. Field of the Invention
One embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device. Note that one embodiment of the present invention is not limited thereto. That is, one embodiment of the invention relates to an object, a method, a manufacturing method, or a driving method. One embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, other examples of embodiments of the present invention include a semiconductor device, a display device, and a liquid crystal display device.
2. Description of the Related Art
A light-emitting element including an EL layer between a pair of electrodes (also referred to as an organic EL element) has characteristics such as thinness, light weight, high-speed response to input signals, and low power consumption, and a display including such a light-emitting element has attracted attention as a next-generation flat panel display.
In a light-emitting element, voltage application between a pair of electrodes causes recombination, in an EL layer, of electrons and holes injected from the electrodes, which brings a light-emitting substance (organic compound) contained in the EL layer into an excited state, and then light is emitted in returning from the excited state to the ground state. The excited state can be a singlet excited state (S*) and a triplet excited state (T*). Light emission from a singlet excited state is referred to as fluorescence, and light emission from a triplet excited state is referred to as phosphorescence. The statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3. Since the spectrum of light emitted from a light-emitting substance depends on the light-emitting substance, the use of different types of organic compounds as light-emitting substances makes it possible to obtain light-emitting elements which exhibit various colors.
To achieve full color display of an image on a display, for example, light-emitting elements of at least three colors of red, green, and blue are necessary. Furthermore, the light-emitting elements are required to have low power consumption.
Examples of specific methods to achieve full color display are as follows: so-called side-by-side patterning in which light-emitting elements that emit light of different colors are separately formed; a white color filter method in which a white light-emitting element is used in combination with a color filter as an optical element; and a color conversion method in which a light-emitting element that emits monochromatic light, such as a blue light-emitting element, is used in combination with a color conversion filter. Each of the methods has advantages and disadvantages.